1. Technical Field
The present invention relates to a controllable assembly of current sources, and in particular to such a controllable current source assembly controlling an amplifier having a gain varying exponentially as a response to an analog control voltage.
Such an amplifier may be formed as an integrated circuit and is for example used as a gain scan amplifier in a mobile telephony equipment.
2. Description of the Related Art
FIG. 1 schematically shows a conventional amplifier, the gain of which exponentially varies as a response to an analog control voltage. Such an amplifier is described in patent U.S. Pat. No. 5,077,541. The amplifier includes an input terminal E receiving a variable positive input voltage Vin. The amplifier further includes two control terminals A and B receiving a control voltage VAB, and an output terminal generating an output voltage Vout. The amplifier includes an attenuator block 2. Attenuator block 2 has an input terminal connected to terminal E, and n output terminals Oi (i ranging between 1 and n). Each output terminal Oi of block 2 generates a control voltage equal to input voltage Vin attenuated according to a predetermined ratio, for example 2i in the case of an R/2R attenuator. Each output terminal Oi of block 2 is associated with a transconductance element gmi controllable by a current. Each transconductance element gmi receives as an input the voltage generated by the terminal Oi of same rank i. An output block 4, connected to a supply voltage VDD, generates voltage Vout as a response to the sum of the currents provided by transconductance elements gmi. Output block 4 is also connected to provide a feedback signal to transconductance elements gmi. A controllable set of current sources 6 includes outputs Si, each output Si being connected to control the transconductances of the transconductance element gmi of same rank i. Control terminals A and B are connected to controllable current source assembly 6.
FIG. 2 schematically shows a conventional controllable current source assembly 6, also described in patent U.S. Pat. No. 5,077,541. Each output terminal Si is connected to the collector of an NPN-type bipolar transistor T1i. The emitters of transistors T1i are interconnected and connected to ground (GND) via a constant current source CS1. The base of each transistor T1i is connected to a node Ni of a control means 8. In control means 8:
the first node Nl is coupled to terminal A;
the last node Nn is coupled to terminal B;
each node Nj (j ranging between 1 and nxe2x88x921) is connected to node Nj+1 via a resistor 10; and
each node Nj (j ranging between 2 and nxe2x88x921) is connected to a first terminal of a constant current source 12 specific to this node.
FIG. 3 illustrates in a simplified manner the variation of the gain of the amplifier of FIG. 1 when the successive transconductance elements gmi are activated. In the example shown, it is assumed that n=6. If each of transconductance elements gm6 to gm1 is successively and separately activated, output block 4 receives a current successively proportional to an attenuation of ratio 26, 25, 24, 23, 22, and 21 of voltage Vin. The amplifier gain thus varies exponentially. The all-or-nothing activation of transconductance elements gmi causes abrupt variations of the gain, according to a stepped curve such as the curve shown in full line. Current source assembly 6 is provided to suppress these abrupt gain variations and provide the gain variation illustrated in dotted lines in FIG. 3.
FIG. 4 schematically illustrates the control currents Ii respectively generated by output terminals Si of current source assembly 6. Control means 8 is provided, when control voltage VAB describes a predetermined voltage range, for successively progressively turning on each transistor T1i, then progressively turning it off, while having the sum of the currents Ii running through all transistors T1i remain substantially constant. It is considered hereafter that control means 8 is provided so that at most two transistors T1i are on at the same time. It is assumed that the preceding predetermined voltage range is included between a minimum voltagexe2x88x92Vmax (VA=0 and VB=Vmax), and a maximum voltage+Vmax (VA=Vmax and VB=0). When voltage VAB is minimum, the voltages of nodes Ni are such that only transistor T16 is on. Transistor T16 is then run through by a maximum current of value Imax. When voltage VAB linearly increases, the voltage of nodes N6, N5 varies so that transistor T16 is progressively turned off while transistor T15 is progressively turned on. The other transistors T1i are off. When transistor T15 is completely on, it is run through by a maximum current of value Imax and transistor T16 is off. When voltage VAB keeps on increasing until value+Vmax, transistor T15 is progressively turned off while transistor T14 is progressively turned on, and so on until transistor T11 is on and run through by current Imax.
However, current sources 12 of control means 8 permanently provide a current in the bases of transistors T1i of current source assembly 6. The existence of these base currents makes the forming of control means 8 difficult, especially for the determination of resistances 10 and of the values of current sources 12. Further, these base currents especially depend on the manufacturing process and on the operating temperature. This results in that a same control voltage VAB will not have the same effect upon current sources 6 for two different operating temperatures.
A completely differential variable-gain amplifier receiving a differential input signal and generating a differential output signal is conventionally formed by means of two identical amplifiers. A first amplifier receives a voltage Vin and generates as a response a voltage Vout, and the second amplifier receives a voltage Vinxe2x80x2 and generates as a response a voltage Voutxe2x80x2. The two amplifiers must be identical so that the output signal is not distorted. It is in particular important that the current sources 6 controlling each amplifier be identical and generate identical control currents for a same control voltage VAB. Now, it is difficult to form two matched current sources 6. In particular, when the amplifier is formed as an integrated circuit, the two sets of transistors T1i may be remote from each other, and it is difficult to form two sets of transistors T1i having the same properties.
Further, the structure of current source assembly 6 is such that its output terminals Si inevitably have a high operating voltage (for example, greater than 2.2 V), due, in particular, to the fact that a high operating voltage (2.5 V) must be provided on control nodes A and B of means 8. This constraint and the existence of the other circuits constitutive of the amplifier of FIG. 1 results in that such an amplifier must be supplied under a high voltage, generally 5 V. Many electronic devices have a supply voltage smaller than 3 V, especially mobile telephony equipment, and it is desirable to have a variable-gain amplifier which can be supplied by a voltage smaller than 3 V.
An object of the present invention is to provide a variable-gain amplifier having the same functions as the amplifier of FIG. 1 and that can be supplied by a voltage lower than 3 V.
Another object of the present invention is to provide such a current source assembly including a control means which is easy to form.
Another object of the present invention is to provide such a current source assembly, having a response to temperature which does not depend on temperature.
Another object of the present invention is to provide such a current source assembly which can provide matched control current pairs.
To achieve these objects, the present invention provides a controllable assembly of current sources including several first output terminals, a first transistor associated with each first output terminal, the current on each first output terminal depending on the current flowing through the first transistor, and a control means provided for, as a response to a predetermined variation of a control voltage, successively progressively turning on, then progressively turning off, each first transistor, in which the first transistors are MOS transistors, and in which each first output terminal is associated with a current mirror formed of MOS transistors, said current mirror providing to the first output terminal a current depending on the current flowing through the first transistor.
According to an embodiment of the present invention, the current mirror associated with each first output terminal includes a second transistor connected in series with the first transistor associated with the first output terminal, and a third transistor connected as a current mirror with the second transistor, the third transistor being also connected so that the current provided by the first output terminal is equal to the current flowing through the third transistor.
According to an embodiment of the present invention, the first transistor is a P-channel transistor having its source connected to a first supply voltage via a constant current source and having its gate connected to a node of the control means, the second transistor is an N-channel transistor having its source connected to a second supply voltage, having its gate connected to the drain, and having its drain connected to the drain of the first transistor, the third transistor is an N-channel transistor having its source connected to the second supply voltage, having its gate connected to the gate of the second transistor and having its drain connected to the first output terminal.
According to an embodiment of the present invention, the current source assembly includes for each of the current mirrors a fourth transistor identical to the third transistor, also connected as a current mirror with the second transistor, and a second output terminal coupled to the fourth transistor so that the second output terminal generates a current equal to the current running through the fourth transistor.
The present invention also aims at a variable-gain amplifier including an attenuator block having an input terminal receiving an input voltage and having several output terminals, several transconductance elements controllable by a current, each transconductance element having an input connected to an output terminal of the attenuator block, a current source assembly such as previously described, each first output terminal of the current source assembly being connected to provide a control current to a transconductance element, and an output block providing an output voltage as a response to the sum of the currents provided by the transconductance elements.
According to an embodiment of the present invention, the variable-gain amplifier includes a differential attenuator block having an input receiving a differential input signal and having several differential output terminals, several pairs of transconductance elements controllable by a current, each pair of transconductance elements having a differential input connected to a differential output terminal of the attenuator block, each pair of transconductance elements generating a current pair, an assembly of current sources such as previously described, having their first and second output terminals connected to, each, control a pair of transconductance elements, and an output block providing a differential output signal as a response to the sum of the current pairs provided by the pairs of transconductance elements.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in conjunction with the accompanying drawings.